1. Field of the Invention
The present invention relates to a radiation delivery system, and in particular to a radiation delivery system used in the non-invasive treatment of human body tissue.
2. Description of the Prior Art
Systems which employ an external radiation source to modify internal body tissue are well known. Examples of such systems include radiotherapy systems, in which an external source delivers nuclear radiation to the body for example to destroy tumors; and ultrasound systems, in which an external source delivers ultrasound to the body for example to destroy kidney stones.
Problems with such systems are that it is often difficult to accurately locate an internal target site which is to receive the radiation and that it is sometimes difficult to maintain delivery of the radiation to that site. These problems may be particularly acute when the site itself is cont inuously moving relative to the body and to the radiation source. This can occur when the site is located on a moving organ such as the heart. In order for the physician to be sure that the internal target site receives a useful dose of radiation an area of tissue much larger than the site itself often may be irradiated. This can result in unnecessary damage to healthy tissue.
A further problem is that any unexpected movement of the body may result in the target site receiving less than the expected dose of radiation.
One known device which attempts to address at least some of these problems is described in the European Patent 0 400 196. This document discloses a radiation delivery system in which an ultrasound source is linked to an X-ray device. The X-ray device is used to locate the target site within the body and to provide information to control the delivery of ultrasound to that site. At least two X-ray images are required each time 3-dimensional position information is needed. Thus both the physician and the patient may be exposed to potentially hazardous ionizing radiation. This may be particularly problematical if the system were to be employed to monitor a movable target site as this is likely to require a large number of X-ray images to be made during a single procedure.